The present invention relates to a photoelectric conversion device and an image sensor and, more particularly, to a photoelectric conversion device and an image sensor having an improved fixed pattern noise characteristics.
Recently, a one- or two-dimensional photoelectric conversion devices having a plurality of photodetectors and a source follower, for storing photo-charges generated by the photodetectors in gate of a MOS transistor and outputting voltage signals converted from the photo-charges, integrally formed on a single semiconductor substrate have been developed. There is a photoelectric conversion device which is disclosed in the Japanese Patent Application Laid-Open No. 9-51085, for instance.
As an example of forming photodetectors and a source follower on a single semiconductor substrate, FIG. 4 shows a circuit configuration of a pixel and FIG. 5 shows a cross sectional view taken along a line B-Bxe2x80x2 in FIG. 4. The source follower is configured with an n-channel MOS (nMOS) transistor.
The gain of the source follower, Gsf, is,
Gsf=1/[1+1/(gmxc3x97rds)]xe2x80x83xe2x80x83(1)
gm: mutual conductance of a driving transistor
rds: saturated drain resistance of a load transistor
Therefore, a large gain is obtained when the mutual conductance of a driving transistor is large.
Accordingly, a source follower configured with an nMOS transistor having larger mutual conductance than that of a p-channel MOS (pMOS) transistor is generally used.
In a MOS transistor, when a voltage is applied across a drain and a source while a channel is formed by applying a voltage to a gate, the electric field becomes strong in the vicinity of the drain-side edge of the channel, which sometimes generates new electron-hole pairs due to impact ionization. Most of the carrier generated due to the impact ionization becomes substrate current and absorbed by a reference potential of the semiconductor substrate, however, a part of the carrier recombines. The recombination is accompanied by light emission, and the emitted light further generates new electron-hole pairs in the semiconductor substrate. The carrier generated in this manner becomes stray carrier which diffuses over the semiconductor substrate. When the stray carrier enters the photodetectors, ghost signals are generated in addition to essential signals generated in proportion to incident light. These ghost signals are a primary factor of fixed pattern noise in a photoelectric conversion device.
The measurement result, by the applicants of the present invention, of substrate current and drain current with respect to gate voltage Vg of nMOS and pMOS transistors is shown in FIG. 6. In FIG. 6, an abscissa shows the absolute value of the gate voltage, and an ordinate shows substrate current and drain current. The substrate current flowing in the nMOS transistor is about 104 to 105 larger than that in the pMOS transistor, which indicates that more electron-hole pairs are generated due to impact ionization in the nMOS transistor than in the pMOS transistor. Thus, since the fact that more substrate current flows in the nMOS transistor than in the pMOS transistor, stray carrier is more easily generated in a semiconductor substrate of an nMOS transistor than a pMOS transistor.
Further, substrate current in a MOS transistor depends upon the drain-source voltage more than the gate voltage. Experimental results show that the substrate current increases logarithmically with respect to an increase in the drain-source voltage. Accordingly, it is determined that generation of stray carrier can be reduced by lowering the drain-source voltage.
Fixed pattern noise, with respect to a signal level, caused by stray carrier entering a plurality of photodetectors is not ignorable as sensitivity of the photodetectors improves.
The present invention has been made in consideration of the above situation, and has as its object to reduce fixed pattern noise in a photoelectric conversion device and an image sensor, to reduce fixed pattern noise caused by stray carrier entering photodetectors in a photoelectric conversion device and an image sensor, and to reduce fixed pattern noise caused by stray carrier entering photodetectors in a photoelectric conversion device and an image sensor having a plurality of photodetectors and a source follower, storing photo-charges generated by the photodetectors in gate of a MOS transistor and outputting voltage signals converted from the photo-charges, integrally formed on a single semiconductor substrate.
According to the present invention, the foregoing object is attained by providing a photoelectric conversion device comprising: a photodetector which outputs charge in accordance with quantity of incident light; and an output circuit having a p-channel MOS transistor for outputting a signal corresponding to the charge from the photodetector, wherein the photodetector and the output circuit is formed on a single semiconductor substrate.
Further, the foregoing object is also attained by providing an image sensor comprising: a photodetector which outputs charge in accordance with quantity of incident light; and an output circuit having a p-channel MOS transistor for outputting a signal corresponding to the charge from the photodetector, wherein the photodetector and the output circuit is formed on a single semiconductor substrate.
With the above configurations, fixed pattern noise is reduced.
According to the present invention, the foregoing object is attained by providing a photoelectric conversion device comprising: a photodetector which outputs charge in accordance with quantity of incident light; and an output circuit having a p-channel MOS transistor for outputting a signal corresponding to the charge from the photodetector in order to restrain generation of stray carrier, wherein the photodetector and the output circuit is formed on a single semiconductor substrate.
Further, the foregoing object is also attained by providing an image sensor comprising: a photodetector which outputs charge in accordance with quantity of incident light; and an output circuit having a p-channel MOS transistor for outputting a signal corresponding to the charge from said photodetector in order to restrain generation of stray carrier, wherein said photodetector and said output circuit is formed on a single semiconductor substrate.
With the above configurations, generation of stray carrier is restrained by configuring the output circuit, which is the source of the stray carrier, with the p-channel MOS transistor, fixed pattern noise caused by the stray carrier entering the photodetector is reduced.
According to the present invention, the foregoing object is also attained by providing a photoelectric conversion device comprising: a photodetector which outputs charge in accordance with quantity of incident light; and a source follower, configured with a p-channel MOS transistor, for converting the charge outputted from the photodetector into voltage, wherein the photodetector and the source follower are formed on a single semiconductor substrate.
According to the present invention, the foregoing object is also attained by providing an image sensor having a plurality of photoelectric conversion devices formed on a single semiconductor substrate, wherein each photoelectric conversion device comprises: a photodetector which outputs charge in accordance with quantity of incident light; and a source follower, configured with a p-channel MOS transistor, for converting the charge outputted from the photodetector into voltage, wherein the photodetector and the source follower are formed on a single semiconductor substrate.
With the configuration as described above, by configuring a source follower which is a main source of stray carrier with a pMOS transistor, generation of stray carrier is restrained, thereby fixed pattern noise caused by the stray carrier entering the photodetectors is reduced.
Preferably, the source follower includes two p-channel MOS transistors connected in series, and the charge outputted from the photodetector is applied to a gate of the p-channel MOS transistor on the ground side, and the voltage is outputted from a node between the two p-channel MOS transistors. Further, an absolute value of a threshold voltage of the p-channel MOS transistor on the ground side is set lower than that of the p-channel MOS transistor on the power source side.
With the above configuration, gain of the source follower is increased.
Further, the p-channel MOS transistor on the power source side is formed on an n-type well whose impurity concentration is higher than that of an n-type semiconductor substrate, and the p-channel MOS transistor on the ground side is formed on the n-type semiconductor substrate outside of the n-type well.
With the above configuration, channels of pMOS transistors, having different threshold voltages, forming a source follower are formed in a single manufacturing step.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.